A CPP-GMR head is considered as one promising sensor to replace the conventional CIP (current in plane) GMR head for over 100 Gb/in2 recording density. In a typical CPP-GMR sensor, a bottom synthetic spin valve film stack is employed for biasing reasons and a CoFe/NiFe composite free layer is conventionally used following the tradition of CIP-GMR technology. One type of CPP-GMR sensor is called a metallic CPP-GMR that can be represented by the following configuration in which the spacer is a copper layer: Seed/AFM/AP2/Ru/AP1/Cu/free layer/capping layer. GMR spin valve stacks are known to have a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer (spacer). One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The pinned layer may have a synthetic anti-parallel (SyAP) structure wherein an outer AP2 layer is separated from an inner AP1 layer by a coupling layer such as Ru. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, in the CIP sensor, the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the magnetoresistive (MR) ratio is higher for a CPP configuration. An important characteristic of a GMR head is the MR ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher MR ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack can improve the MR ratio and increase sensitivity.
Another type of sensor is a so-called confining current path (CCP) CPP GMR sensor where the current through the Cu spacer is limited by the means of segregating metal path and oxide formation. With a current confining (CCP) scheme, CPP GMR performance can be further improved. An example of a CCP CPP-GMR sensor has the following configuration: Seed/AFM/AP2/Ru/AP1/Cu/CCP layer/Cu/free layer/capping layer where the CCP layer is sandwiched between two copper layers.
In a CPP operation mode, a tunnel magnetoresistive (TMR) head is another candidate for realizing high sensitivity. In this design, the non-magnetic conductive layer between the pinned layer and free layer in the GMR stack is replaced by an insulating layer such as AlOx or MgO. When the magnetoresistive element is a magnetic tunnel junction (MTJ), the tunneling (insulating) layer may be thinned to give a very low RA (<5 ohms-μm2).
Heusler alloys such as Co2MnX (X is Si, Ge, Al, etc.) have attracted much interest due to their large spin polarizations and high Curie temperatures. A very large magnetoresistance (MR) ratio has been observed in TMR multilayer structures with Heusler alloys, especially with Co2MnSi, as indicated in the following four references: S. Kammerer et al, Appl. Phys. Lett. 85 (2004) 79; Y. Sakuraba et al, Japanese Journal of Applied Physics, Vol. 44, No. 35, pp. L1100-L1102 (2005); S. Okamura et al, Applied Phys. Lett., 86 (2005) 232503; and Y. Sakuraba et al, Applied Phys. Lett., 88 (2006) 022503. With this published data, those skilled in the art could easily predict that a large GMR ratio could also be achieved in a CPP-GMR sensor when a Co2MnSi layer is employed as an AP1 layer or free layer. However, the condition needed to achieve an ordered half metal structure for Co2MnSi is very difficult and typically requires substrate heating as well as a lengthy high temperature post-annealing treatment. These post-annealing processes generally require a temperature above 350° C. which would destroy the underlying shield structures (in a TMR head) and severely degrade the pinning strength thereby hindering Heusler alloys from practical GMR or TMR sensor applications.
The spin polarization of Co2MnSi is very sensitive to its site-disordering states due to its unique band structure. Therefore, in order to realize the full potential of the Co2MnSi spin polarization, it is necessary to remove the site-disorder states and the defects by means of high temperature annealing. For example, the as-deposited single Co2MnSi layer displays no magnetic moment, no spin polarization, and a very large resistivity which indicates that the film is amorphous. However, after annealing at 350° C. for 5 hours, a magnetic moment begins to appear and the resistivity is reduced considerably, indicating the film is crystalline. It follows that if a Co2MnSi film were applied directly on top of the bottom shield in a TMR sensor and annealed at high temperature in order to achieve the desired MR ratio, then the bottom shield would be stressed to the point where its domain would enlarge substantially and its surface roughness would increase severely with loss of built-in patterns. These conditions would lead to a very poor spin valve performance. Therefore, a new method is needed to incorporate a Heusler alloy such as Co2MnSi in a spin valve structure without negatively affecting the underlying substrate or the pinning strength within the sensor.
A CPP-GMR head is generally preferred over a TMR head design for ultra-high density recording because the former has lower impedance. However, the resistance (RA) in a conventional single spin valve is too small (<100 mohm-μm2) and the MR ratio of a CPP head may be very low (<5%). Additionally, the output voltage which is related to the resistance change is unacceptably low for many CPP-GMR configurations. One way to increase the resistance change is to optimize the materials and structure of the CPP-GMR head. In particular, it is desirable to modify the pinned layer and/or free layer to improve performance.
In U.S. Pat. No. 6,876,522, a ferromagnetic Heusler alloy (Co2MnSi or Co2MnGe) is used in combination with a non-magnetic spacer Heusler alloy such as Rh2CuSn or Co2CuSn. U.S. Pat. No. 7,023,670 discloses a metalloid ferromagnetic Heusler alloy layer between a non-magnetic material layer and a free magnetic layer and between a pinned magnetic layer and the non-magnetic material layer.
U.S. Patent Application US 2003/0137785 describes a magnetic sensing element in which a portion of the pinned layer adjacent to a non-metallic spacer is comprised of a Heusler alloy sandwiched between two conventional magnetic materials such as CoFe to prevent diffusion of Mn from the Heusler alloy into the non-metallic spacer.
U.S. Pat. No. 6,977,801 and related U.S. Patent Application US 2004/0165320 disclose a tunnel junction wherein a ferromagnetic layer such as FeCo is inserted between an AFM layer and a pinned layer made of a Heusler alloy.
In U.S. Patent Application US 2003/0116426, a method of co-sputtering a Heusler alloy is described in which each of the components is sputtered as a single target.
U.S. Pat. No. 7,038,894 shows a double tunnel junction in which a free layer formed between two insulating layers has a laminated structure comprised of alternating non-magnetic layers (Al or Cr) and Heusler alloy layers that are anti-parallel coupled.